1. Field of the Invention
The present invention relates to low noise converters, particularly a low noise converter of a satellite broadcasting receiver.
2. Description of the Background Art
FIG. 8 represents a typical satellite broadcasting reception system.
Referring to FIG. 8, a low noise converter (LNB: Low Noise Block Down Converter) 102 is attached to an antenna 101. Antenna 101 receives incoming signals having a frequency in the 12 GHz band (11.71 GHz-12.01 GHz) from a broadcasting satellite 110.
Low noise converter 102 frequency-converts an incoming weak radio wave from broadcasting satellite 110 into an IF signal of the 1 GHz band and applies low-noise amplification for supply to the so-called STB (Set Top Box: tuner) 104 that is connected. A signal of low noise and sufficient level by virtue of low noise converter 102 is supplied to STB 104. STB 104 has the IF signal supplied from a coaxial cable 103 processed by the internal circuit for supply to a television 105.
FIG. 9 is a block diagram representing an example of the circuit of a general low noise converter.
Referring to FIG. 9, an incoming signal of the 12 GHz band is received at an antenna probe 213 in a feed horn (waveguide), subjected to low-noise amplification at a low noise amplifier 204 (LNA), and then passed through a band pass filter (BPF) 206 that serves to pass through a signal of the desired frequency band and remove any signal of the image frequency band. The signal passed through band pass filter 206 is mixed by a mixer 208 with a local oscillation signal of 10.678 GHz from a local oscillator 218 to be converted into an intermediate frequency signal of the 1 GHz band, i.e. frequency-converted into an intermediate frequency (IF) signal.
The output from mixer 208 is amplified at an intermediate frequency amplifier 210 to have an appropriate noise property and gain property for transmission to a terminal P1 via a capacitor 212. The low noise converter has a power supply circuit 220 provided for receiving DC voltage via an inductor 214 to supply the required power supply current or bias voltage to LNA 204, local oscillator 218 and intermediate frequency amplifier 210. For the purpose of preventing the IF signal from entering the power supply system, inductor 214 and capacitor 216 are connected at a connection node of terminal P1 and power supply circuit 220.
One known type of a low noise converter is a universal LNB having incoming signals of two bands differing in frequency from a broadcasting satellite, frequency-converted at local oscillators corresponding to two different types of frequencies for using a switched one, and receiving two types of polarized waves (horizontal polarized wave (H polarized wave), vertical polarized wave (V polarized wave)) of incoming signals by switching an LNA first-stage element.
FIG. 10 is a block diagram representing an example of a circuit of a universal low noise converter.
Referring to FIG. 10, a universal low noise converter includes a feed horn 202#, an LNA 204# selectively amplifying a V polarized wave signal and H polarized wave signal applied from feed horn 202#, a band pass filter 206 limiting the band of the output from LNA 204#, local oscillators 218A and 218B, a mixer 208 mixing a local oscillation signal from local oscillators 218A and 218B with the output from band pass filter 206 for conversion into an intermediate frequency, an intermediate frequency amplifier 210 amplifying the output of mixer 208, and a capacitor 212 coupled between the output of intermediate frequency amplifier 210 and terminal P1.
The universal low noise converter further includes an inductor 214 for transmitting DC voltage, and a power supply circuit 220# receiving DC voltage via inductor 214 to supply power to LNA 204#, local oscillators 218A and 218B, and intermediate frequency amplifier 210. For the purpose of preventing the IF signal from entering the power supply system, inductor 214 and capacitor 216 are connected at the connection node of terminal P1 and power supply circuit 220#.
LNA 204# includes an amplification circuit 205A amplifying a V polarized wave signal, an amplification circuit 205B amplifying an H polarized wave signal, and an amplification circuit 205C having its input coupled to the outputs of amplification circuits 205A and 205B.
Local oscillator 218A outputs a first local oscillation signal (9.75 GHz). Local oscillator 218B outputs a second local oscillation signal (10.6 GHz) having a frequency higher than that of local oscillator 218A.
Power supply circuit 220# selectively supplies voltage to amplification circuit 205A or 205B depending upon whether an in-door tuner receives a V polarized wave signal or an H polarized wave signal. The switching of high/low frequency of the local oscillator is allowed by selectively supplying voltage to local oscillator 218A or 218B.
There is also a low noise converter having two or more output terminals, and a circuit that can output an arbitrary or fixed signal from each of the output terminals.
FIG. 11 is a block diagram representing an example of a universal twin low noise converter.
Referring to FIG. 11, a universal twin low noise converter includes a feed horn 203, a frequency conversion unit applying frequency conversion to an H polarized wave signal received by feed horn 203 to output two IF signals, and a frequency conversion unit to apply frequency conversion to a V polarized wave signal received by feed horn 203 to output two IF signals.
Specifically, the frequency conversion unit applying frequency conversion to an H polarized wave signal includes an LNA 204A amplifying an H polarized wave signal received by feed horn 203, and a distributor 207A dividing the output from LNA 204A into two.
The frequency conversion unit applying frequency conversion to an H polarized wave signal further includes band pass filters 206A and 206B removing image signals from the two outputs of distributor 207A, local oscillators 218A and 218B, mixers 208A and 208B mixing a local oscillation signal output from local oscillators 218A and 218B (signal of frequency 9.75 GHz and signal of frequency 10.6 GHz) with the outputs of band pass filters 206A and 206B, respectively, and intermediate frequency amplifiers 209A and 209B amplifying IF signals of an intermediate frequency band from mixers 208A and 208B, respectively.
The frequency conversion unit applying frequency conversion to a V polarized wave signal includes an LNA 204B amplifying a V polarized wave signal received at feed horn 203, and a distributor 207B dividing the output of LNA 204B into two.
The frequency conversion unit applying frequency conversion to a V polarized wave signal further includes band pass filters 206C and 206D removing image signals from the two outputs of distributor 207B, mixers 208C and 208D mixing a local oscillation signals output from both of local oscillators 218A and 218B with the outputs of band pass filters 206C and 206D, and intermediate frequency amplifiers 209C and 209D amplifying IF signals of intermediate frequency band output from mixers 208C and 208D, respectively.
The universal twin low noise converter is also provided with a radio frequency selection circuit 225 selecting two from the four IF signals that are frequency-converted versions of the H polarized wave signal and V polarized wave signal output from the frequency conversion unit for output.
The universal twin low noise converter further includes intermediate frequency amplifiers 210A and 210B amplifying an IF signal of intermediate frequency band output from radio frequency selection circuit 225, capacitors 212A and 212B transmitting the outputs of intermediate frequency amplifiers 210A and 210B to terminals P1 and P2, respectively, inductors 214A and 214B for transmitting DC voltage, and capacitors 216A and 216B.
The universal twin low noise converter further includes a power supply circuit & switch control unit 221. Power supply circuit & switch control unit 221 receives voltage from terminals P1 and P2 to deliver power supply voltage to each circuit in the low noise converter and carries out a command to switch the output with respect to radio frequency selection circuit 225.
Specifically, voltage is supplied to an LNA group 205 formed of LNAs 204A and 204B, a first intermediate frequency amplification unit 209 formed of local oscillators 218A and 218B and intermediate frequency amplifiers 209A-209D, and a second intermediate frequency amplification unit 211 formed of intermediate frequency amplifiers 210A and 210B.
The signal to be output to each terminal is selected by radio frequency selection circuit 225 and power supply circuit & switch control unit 221. Specifically selection between a high band (frequency band of 1100-2150 MHz) and a low band (frequency band of 950-1950 MHz) and selection between an H polarized wave signal and a V polarized wave signal are carried out by radio frequency selection circuit 225 and power supply circuit & switch control unit 221.
In this universal twin low noise converter, the switching scheme of incoming signals from a satellite was realized using generally a 4-input 2-output switch IC (for example, the aforementioned radio frequency selection circuit 225).
However, the recent years have seen the presence of an IC that implements the functions of a mixer, a local oscillator, and an intermediate frequency amplifier on one chip. Therefore, the possibility of mounting a switching circuit on a microwave signal circuit of 12 GHz is now enhanced to be adapted to practical use.
As technical documents of the switching circuit art, Japanese Patent Laying-Open No. 2001-168751 (PTL 1) and Japanese Patent Laying-Open No. 8-293812 (PTL 2) can be cited.
PTL 1 discloses a satellite broadcasting reception system, a low noise block down converter and a satellite broadcasting receiver employed in the satellite broadcasting reception system, as set forth below. Specifically, the low noise block down converter includes a conversion unit receiving a plurality of types of polarized wave signals transmitted from each of a plurality of satellites to convert the plurality of polarized wave signals into a plurality of intermediate frequency signals, an amplification switch connected to the conversion unit, having a plurality of outputs connected to a plurality of output ports, respectively, receiving a plurality of intermediate frequency signals, and determining the status according to a selection signal for providing amplified intermediate frequency signals, and a first control unit receiving externally applied digital serial data for selecting a satellite via an output port, and providing a selection signal based on the digital serial data.
PTL 2 discloses a switch circuit of a converter for satellite broadcasting (wireless reception device). The switch circuit of a satellite broadcasting converter switches a plurality of local oscillators incorporated in the satellite broadcasting converter and having a plurality of oscillation frequencies according to a pulse signal overlapped with a band switching pulse signal sent out from a satellite broadcasting tuner. The switch circuit includes a filter circuit receiving a pulse signal from the satellite broadcasting tuner for extracting only the frequency component of the band switching pulse signal, an amplification circuit amplifying the pulse signal from the filter circuit, a rectifying circuit rectifying the pulse signal amplified by the amplification circuit, a comparison circuit comparing the DC voltage from the rectifying circuit with a reference voltage to output a signal indicating whether the pulse signal is overlapped with a band switching pulse signal, and a drive circuit receiving a signal from the comparison circuit to drive a local oscillator of an oscillation frequency corresponding to the compared result.
To realize a switching circuit at a microwave signal circuit corresponding to the frequency band of 12 GHz, conventionally a plurality of amplification circuits employing high electron mobility transistors (HEMT) are connected in parallel, which are respectively turned ON/OFF to switch (select) a signal (universal single LNB, or the like).
In the case where the above-described switching method is to be applied to a universal twin low noise converter, signal switching is conducted by turning ON/OFF the succeeding-stage amplification circuits of the microwave signal amplification circuit. Particularly in the case where an HEMT is employed as the amplification element in the succeeding-stage amplification circuit, the signal could not be completely cut even if the HEMT is OFF. Due to signal leakage, the desired isolation property could not be achieved.
The usage of a PIN (P-Intrinsic-n) diode is known to realize signal selection. A PIN diode corresponding to a microwave signal related to the frequency band of 12 GHz is so expensive that it cannot be readily employed for general consumer products.
In view of the foregoing, an object of the present invention is to provide a low noise converter that can be readily realized at low cost without degrading the desired isolation property when signal switching of a universal twin low noise converter is implemented by switching the ON/OFF state of a microwave amplification circuit of 12 GHz.